Add request validation layer for all endpoints

Introduce a validation layer for request bodies, params, and query strings using a validation library. Return structured 4xx errors that include which fields failed validation and why.

Look at the existing endpoints in the API routes to understand the current request handling. Choose a validation library (like Joi, Zod, or class-validator) and integrate it into the request pipeline. Ensure validation errors return structured 4xx responses with field-specific messages. Test by sending invalid requests to endpoints.

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